1. Field of the Invention
The present invention relates to a synthetic leather using, in a synthetic resin layer which is the base material of the synthetic leather, cross-linked foamed syndiotactic 1, 2-polybutadiene. The synthetic leather has favorable softness providing the touch and aesthetic properties of natural leather, undergoes low changes in flexibility within the temperature range of ordinary use, and having good elasticity.
2. Description of the Prior Art
Most synthetic leathers have a multi-layered structure comprising a base fabric layer, an adhesive boundary layer, a resin layer and a top layer. In the resin layer, quasi-natural leathers such as foamed or unfoamed polyvinyl chloride, polyamides, polyurethanes, polyamino acids, etc. are conventionally used. Those which contain a base sheet comprising polyvinyl chloride are generally called vinyl leathers, and the foamed products thereof are called sponge leathers to discriminate them from oridinary synthetic leathers. Vinyl leathers have the practical advantages that they are comparatively inexpensive and can be produced with ease, and they enable one to obtain suitable physical properties for the artificial leather. Therefore, they are currently used in the largest amount neglecting various defects with respect to physical properties, i.e., since polyvinyl chloride is an essentially glassy and hard resin, a large amount of a plasticizer must be compounded therewith in order to convert it to a leather or like soft material. In particular, for high-grade leathers which require good softness through all seasons, dioctyl phthalate, di-n-octyl phthalate, dioctyl sebacate or like plasticizers are mainly compounded therewith. These plasticizers are partly vaporized away during production of the synthetic leather and cause the phenomenon of bleeding, migration, etc., thus being unavoidably released from the base material to come into contact with the human body. Such a release of plasticizer can be the cause of a reduction in flexural fatigue resistance at low temperatures in winter and blocking at high temperatures in summer. Recently, the harmful effects of many plasticizers have been suspected. From this viewpoint, there have been conducted various investigations of the use of foamed products of plasticizer-free high pressure process polyethylene or ethylene-vinyl acetate copolymers containing vinyl acetate in high proportions. However, these are so poor in adhesive properties that adhesion to a base fabric layer or to a top layer is difficult. Further, they are inferior in softness. Thus, they have not yet replaced conventional vinyl leathers.